<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <script>
        alert("2"==2);
        alert("10"-2);

        var alist = [1,2,3,4,4,3,2,1,2,3,4,5,6,5,5,3,3,4,2,1];
        var alist2 = [];
        alert(alist.length)
        for (var i = 0; i < alist.length;i++)
        {
            if (alist.indexOf(alist[i]) == i) {
                alist2.push(alist[i]);
            }
        }
        alert(alist2)


        var alist = [1,2,1,3,5]
        alert(alist.indexOf(1))


        var p1=null,p2=undefined;
        alert(p1==p2)
        alert(p1===p2)
    </script>
</body>
</html>